The present invention relates to a feedback system employing a sense amplifier, and more particularly to a sense amplifier circuit having improved common mode rejection.
There are many applications of electronic circuits that involve a sensor and a feedback configuration in which the sensed signal is employed to perform error compensation or a similar function. One particular application for a sensor and feedback circuit is a motor controller, such as for a disk drive actuator. This type of system employs a power amplifier for delivering a current to drive the actuator motor, a series sense resistor to sense the drive current, and a sense amplifier feeding back the sensed drive current to the power amplifier to perform error compensation. An exemplary configuration of such a system is shown in FIG. 1.
The feedback system of FIG. 1 has a number of parameters that must be controlled in order to achieve desired performance. The sense amplifier must be designed to provide a precise amount of gain to the sensed signal across the sense resistor. This gain must remain constant independent of the common mode signal, which may be orders of magnitude larger than the desired signal. In the system of FIG. 1, a higher bandwidth of sensing and error correction than that of the load (actuator motor) may be desired. The increased bandwidth is achieved by the error amplifier and compensation circuit connected to receive the feedback from the sense amplifier. This compensation may cause the common mode signal to be much larger than the desired signal across the sense resistor at frequencies above the load cutoff frequency. The sense amplifier must be able to reject this common mode signal, or inaccuracies and/or instability of the system may result.
Previous efforts to provide an effective sense amplifier feedback circuit have experienced a number of practical difficulties which limit their effectiveness. A classic solution has been to employ an operational amplifier with resistive feedback. However, the performance of this circuit depends on the ability to match the circuit""s resistors precisely, and it is not possible to achieve such precise matching in a circuit implemented in silicon. As a result, the common mode rejection of this circuit is limited. Another solution that has been attempted is a single-ended transconductance amplifier circuit. This type of circuit is not able to linearly amplify signals, some of which may extend below ground by up to a diode voltage drop, and thus cannot achieve the precise gain required of a sense amplifier in a motor controller circuit such as is shown in FIG. 1.
There is a continuing need in the art for an improved motor controller circuit employing a sense amplifier feedback circuit with precise gain and outstanding common mode rejection. Such a circuit is the subject of the present invention.
The present invention is a sense amplifier having improved common mode rejection. The sense amplifier has a differential input and a differential output. A first level shifting transconductance circuit is connected to receive the differential input. A gain and compensation circuit is connected to the level shifting transconductance circuit, and a buffer is connected to the gain and compensation circuit. The differential output of the sense amplifier is taken at an output of the buffer. A feedback network is connected between the output of the buffer and an input of the gain and compensation circuit. The feedback network includes a divider circuit connected to the output of the buffer and a second level shifting transconductance circuit connected between the divider circuit and the input of the gain and compensation circuit. The first and second level shifting transconductance circuits are preferably matched to one another for distortion cancellation.